Thanks for all the comments here which are very helpful. Perhaps naively I had thought there would be a simple rule of thumb but it seems not. That alone helps. Perhaps I need to persevere and I am thinking that the discomfort I felt with the previous dual monitor set up was that I didn't use it for long enough. I had the monitors side by side and perhaps having them at an angle to one another will help also.
I do have the Tangent panels so need the depth to accommodate them and a key board for, in the main, Capture One and Photoshop. I also want to be able to use the space for matting prints from time to time.
That lead me to a depth from front of desk to the wall of around 40 inches. I can always move the monitors forward if necessary but not backwards of course. If, over time, I move to a 4k/5k monitor then that may need to be positioned differently but i'd rather risk having too much space than too little.
Thanks again for your comments and suggestions.